Apparatus for heating by microwave energy

ABSTRACT

An apparatus for heating by microwave energy includes a dielectric material having low losses and a dielectric constant exceeding the dielectric constant of air. The dielectric material has a substantially constant cross-sectional dimension in the direction of propagation of the microwave energy, and has a thickness selected such that phase velocity therethrough is lower than the phase velocity of light in air, thereby concentrating the energy in the dielectric material adjacent the material to be heated.

United States Patent Berggren et al.

[ Nov. 12, 1974 APPARATUS FOR HEATING BY MICROWAVE ENERGY Inventors: Benny Berggren, Spanga; Goran ,Boling, Taby, both of Sweden;

Marshall N. Dickler Assignee: Stiftelsen lnstitutet for mikrovagsteknik vid Tekniska Hogslrolan i Stockholn, Stockholm Sweden Filed: May 23, 1973 Appl. No.: 362,912

Foreign Application Priority Data May 29, 1972 Sweden 7014 72 U.S. Cl. 219/1055, 333/95 R Int. Cl. H05b 9/06 Field of Search 219/1055; 333/95 R, 95 S [56] References Cited UNITED STATES PATENTS 2,856,497 10/1958 Rudcnberg 219/1055 Primary ExaminerE. A. Goldberg Assistant ExaminerHugh D. Jaeger Attorney, Agent, or Firm-Silverman & Cass [57] ABSTRACT 9 Claims, 10 Drawing Figures E U new 2 I974 sum 1 nr 2 PATENTEUHUV 1 21974 sum 2 or 2 g J \n APPARATUS FOR HEATING BY MICROWAVE ENERGY This invention relates to an apparatus for heating by microwave energy.

One of the greatest problems in heating by microwave energy is to develope an apparatus, which has a really good efficiency with respect to transferring microwave energy from a microwave generator to the object to be heated. This applies particularly to the heating of a continuous material flow or of objects conveyed on belts. The main object of a heating apparatus of that kind would be to concentrate the microwave energy to a volume, within which the object is intended to be placed. One object of the invention is to provide an apparatus, which comprises an easily accessible heating volume and is capable of heating objects of varying geometric shape, and which is adapted for continuous heating of movable objects as well as for heating of stationary objects. A further object of the invention is to provide a heating apparatus, which provides uniform heating within the heating volume. A still further object of the invention is a heating apparatus, by

which a pressing force can be applied to the heated object simultaneously with its heating. The characterizing features of an apparatus according to the invention will be apparent from the attached claims.

The invention is described in greater detail in the following, with reference to the accompanying drawing, in which:

FIG. 1 shows an apparatus with a plane dielectric plate,

FIG. 2 shows a part of an apparatus with two dielectric plates and a diagram illustrating the energy distribution in said plates,

FIG. 3 shows an'apparatus with two plane dielectric plates,

FIG. 4 shows an apparatus with the heating element in the form of a dielectric layer on a cylinder,

FIG. 5 shows an apparatus with a cylindric dielectric body having a metallic core,

FIG. 6 shows an apparatus with a homogenous dielectric body,

FIG. 7 shows an apparatus with a dielectric body shaped as a cylindrical lining in a metal tube,

FIG. 8 is a cross section view taken along the line II of the apparatus shown in FIG. 7,

FIG. 9 shows a cross section view of an apparatus where the dielectric body is a rod, and

FIG. 10 shows a cross section view taken along the line II-II of the apparatus shown in FIG. 9.

The apparatus according to FIG. 1 comprises a waveguide 10, a connecting coil 11, a dielectric plate 12 with small losses, a metallic disc 13 and a termination piece 14 of a material with heavy losses.

On a plane parallel] plate 12 of the aforesaid kind, TM-modes provide a wave propagation of electromagnetic fields, the energy density of which decreases exponentially abovethe plate surface. The decrease is determined by the phase velocity v; of the microwaves which is w/B, in which an is the angular frequency and B is the wave propagation constant. The lower the phase velocity v; is, the more rapid is the decrease. The lowest phase velocity which may occur is 0/ Y, in which c is the velocity oflight in air and Z is the relative dielectric constant of the dielectric material. Thus the limit of the speed at which the decrease may occur, is

determined by the value of the dielectric constant for the material of which the plate 12 is made. The phase velocity is determined also by the thickness of the plate, which is chosen so that the microwaves will have the desired phase velocity between 0 and 0/ Vi when the dielectric constant 2 is given. If the phase velocity v; of the microwaves exceeds the velocity 0 of light in air or is equal thereto, energy radiates away from the plate 12, which under such conditions is not applicable to microwave heating. This can happen for all modes, except the lowest TMmode, the basic mode.

If the plate has a sufficient thickness, two or more modes can propagate simultaneously whereby energy can be transferred between such modes through the object 16 to be heated. The higher modes, which always have a higher phase velocity v,, render a different decrease in energy density and thereby change the energy distribution in a manner not desired. The production of two or more modes is prevented by giving the plate 12 a thickness below 0/2 fl, V Z l where f is the frequency of the microwave energy. When a plate is dimensioned for one application, a higher value of the dielectric constant will result in a thinner plate.

In order to show the relationships between thickness of the dielectric portions and energy distribution across the dielectric material, reference is made to FIG. 2. FIG. 2 shows distribution element 21-23 with a dielectric portion 21 and a metallic portion 23. The dielectric portion has the dielectric constant 2, and the thickness t Further, there is a similar distribution element 22-23 with a dielectric portion 22 having the dielectricity constant 22 El, and the thickness t t and a metallic portion 23. The two elements are totally separate and distinct from one another and are shown together here oly for comparison purposes. For comparison purposes the upper surfaces of both elements are located in the same horizontal plane. In the diagram h-W, in which h is the height above the common horizontal plane and W is the energy, the energy distribution for the dielectric plate 21 of greater thickness is indicated by the dashed curve and for the thinner dielectric plate 22 by the fully drawn curve.

In the apparatus according to FIG. 1 the lower end of the waveguide 10 is connected to a microwave source (not shown) and its upper end is connected via the coil 11 to an L-shaped intermediate portion 15, which together with the disc 13 constitutes a holder means for the plate 12. The L-shaped intermediate portion has also as an object, together with the disc 13, to conduct the microwave energy out onto the plate 12. Thus, the microwave energy which is fed into the waveguide 10 is transferred via the coil 11 and intermediate portion 15 to the plate 12, where it is propagated and supplied to the material 16 placed on the plate and which is intended to be heated. Surplus energy, if any,

is absorbed in the termination piece 14. This termination piece may be completed by a reflector means 17 consisting of metal and placed after the termination piece in the propagation direction. The object of said reflector means is to reduce still more the leakage from the heating apparatus. It is also possible to use a reflector means without the intermediate termination piece 14, and in FIG. 1 such a reflector means has been shown by a dashed line and has the reference 17a.

The microwave energy in the waveguide 10 produces a current through the coil 11. The current in the coil has only a small phase variation, which is at least less than 90. This current produces a surface wave of TM- type in the dielectric plate 12. By the indicated connection type, the wave is given a phase variation less than 90 in the shown z-direction, i.e., only the lowest TM- mode, the basic mode, is excited. Also other types of connection arrangements which would render the same result may be used.

Generally, the microwave energy should be fed in so that the microwaves are not given any phase variation perpendicular to the direction of the wave propagation (x-direction). The reason for this is that the microwave energy is to be coupled as strongly as possible to the basic mode, which has no phase variation is said direction. Further, a highly uniform heating is obtained;

The object to be heated is placed adjacent to the surface of the dielectric plate where the energy density is high. The microwave energy is converted to heat by conduction and dipole losses.

7 The apparatus according to FIG. 3 comprises a distribution element 10-12-13 according to FIG. 1 and an additional distribution element 30-32-33 of the same kind as the first mentioned. The two elements are so disposed that the two dielectric portions 12 and 32 face each other at a certain spaced relationship in a plane parallell position relative each other. The microwave energy sourcecan be connected to the waveguide 10 as well as to the waveguide 30, or possibly only to one of them. When one microwave source is used the other dielectric plate acts as a reflector, reflecting energy back towards the object being heated. By this arrangement an extremely uniform heating is obtained. It provides, in addition, the possibility of applying, by means 7 of the plates 12-32, a pressure to the object during the heating.

The apparatus according to FIG. 4 comprises a metal cylinder 44 carried rotatably in bearings by a shaft 44a and having a dielectric layer 47. The material 46 to be heated has for example the shape of a plate and is supplied immediately adjacent to the dielectric layer 47 on the cylinder. In such cases where a pressure is required together with the heating of the material 46, for example when wood is to be glued, the cylinder 44 with the dielectric layer 47 is pressed against the material 46, and the last mentioned will then put the cylinder 44-47 in rotation. The same is the case when paper or other materials in the form of a running line or web are heated, where the periphery of the cylinder 44-47 is entirely or partly enclosed by the material line. The microwave energy is kept substantially constant along the periphery of the cylinder 44-47 until it is absorbed by the material 46. The feeding of the microwave energy to the heating element 44-47 may, for example, take place by means of an element 40-42-43 having in principle the same design is the one shown in FIG. 1, with the components, 40, 42 and 43 corresponding to the components denoted as 10, 12 and 13 respectively in FIG. 1.

'- The apparatus according to FIG. 5 comprises a distribution element in the form of a circular cylinder 52 of dielectric material with small losses and having a core 53 of metal. The microwave source is connected via a coaxial line 50-53. The apparatus may be used, for ex- I ample, for the heating of liquid or powder materials,

The apparatus according to FIG. 6 comprises a circular cylinder 62 of dielectric material with small losses and a cylindric waveguide 60 connected to the microwave source.

The apparatus shown in FIG. 7 and FIG. 8 comprises a metal tube 70, the inner surface of which is lined with a dielectric material 71 having low losses, said dielectric material forming a tube coaxial with the metal tube 70. The tube -71 is mainly intended for heating dielectric material 72 with cylindrical cross section, such material being positioned in the dielectric tube 70-71 in coaxial relationship to said tube. The apparatus may also be used for heating dielectric material in the form of powder or granulate, which is conveyed through the tube 70-71, for example by gravity, after being supplied through the metal tube 73 which, as will be explained later on, also serves for the feeding of the microwave energy.

The feeding of microwave energy to the apparatus shown in FIG. 7 and FIG. 8 may for example take place by means of a coaxial line, the outer conductor of which is the metal tube 70 and the inner conductor is the already mentioned metal tube 73. The microwaves propagate in the direction shown by the arrow in FIG. 7 and in a TM-mode without variations in the -direction (see FIG. 8) so that a rotationally symmetrical field is obtained. The thickness and the value of the dielectric constant of the material 71 with the low losses will influence the configuration of the electromagnetic field. If the losses are known, i.e., if the value of the dielectric constant and the loss angle are known for the material 72 to be heated, the electrical field can be given such a configuration that the heating will be uniform also in the radial direction.

The apparatus according to FIG. 9 and FIG. 10 comprises a rod of dielectric material with small losses. The rod has at least two opposite plane sides. A metal plate 81 is bent in such a way that its surface portions which are obtained by the bending and facing each other engage the entire area of the two opposite plane sides of said rod 80. The cross section of the metal plate 81 outside of the cross section of the rod 80 may have any configuration and has here for the sake of simplicity been shown as an U with right angles between stems and the transverse.

The apparatus according to FIG. 9 and FIG. 10 is intended for the heating of thin strips or strings of material 82, said strings or strips 82 being placed close to that free side of the dielectric rod 80, which is not enclosed by the metal plate 81.

The rod 80 is fed with microwave energy in the longitudinal direction (see the arrow on the rod 80 in FIG. 9) so that the microwaves propagate in a TE-mode. The electrical field is directed between the opposite sides of the metal plate 81 and the greatest part of it is located in said rod. Because the metal plate 81 forms a closed space at one side, no TM-waves can propagate. If the distance b in FIG. 10 is less than one half of the wavelength in the dielectric rod 80, the field strength in a cross section normal to the opposite sides of the metal plate 81 will be constant and a uniform heating of homogenous material will be obtained. The choice of the material in the rod will determine the degree of concentration of the energy to the surface of the rod. The supply of microwave energy to the apparatus is shown in FIGS. 9 and 10 to take place by means of a wave guide 83, but is not limited to this way.

The invention is not limited to the embodyments shown and described above, but comprises all such variations and modifications that fall within the scope of the attached claims, and the heating element is in each case given such a shape that suites the material to be heated.

What we claim is:

1. An apparatus for heating material by microwave energy comprising, a distribution element for receiving the microwave energy and coupling the microwave energy to the material to be heated, said distribution element including a dielectric material having low energy losses and having a dielectric constant exceeding the dielectric constant of air, said dielectric material having a substantially constant cross-sectional thickness at successive points therealong in the direction of propagation of the microwave energy therethrough, said dielectric material further having a thickness for maintaining the phase velocity of said microwave energy therethrough lower than the velocity of light in air, and having a thickness range for maintaining only the basic resonant mode of microwave energy in said dielectric material whereby the microwave energy will be concentrated within and adjacent to said dielectric material, in the immediate vicinity of the material to be heated.

2. An apparatus according to claim 1 wherein said dielectric material maximum thickness is no greater than a thickness determined by the formula T 0/2 f V 2 1 wherein c is the velocity of light in air, f,, is the frequency of the microwave source and E is the relative dielectric constant of the dielectric material.

3. An apparatus according to claim 2 wherein the dielectric material has the form of a plate.

4. An apparatus according to claim 3 wherein said dielectric material has two substantially parallel surfaces forming the top and bottom surfaces respectively of said plate and further including metallic conductor means secured to said bottom surface, said top surface constructed and arranged to be placed in contact with the material to be heated.

5. An apparatus according to claim 4 further including a second distribution element having substantially the same form of said distribution element, said distribution element and said second distribution element being positioned with respect to one another such that said dielectric material top surfaces are spaced apart by a predetermined distance and facing one another forming a plane parallel arrangement.

6. An apparatus according to claim 2 wherein the dielectric material has the form of a cylinder.

7. An apparatus according to claim 6 further including a core of metal positioned within said cylinder.

8. An apparatus according to claim 2 wherein said dielectric material is cylindrical in form having an inner and outer surface, and further including metallic conductor means having a cylindrical form, said cylindrical form of dielectric material and said cylindrical form of said metallic conductor means being axially aligned with said metallic conductor means tightly encasing said dielectric material.

9. An apparatus according to claim 2 wherein said dielectric material is shaped as a rod having first and second plane parallel sides, and further including a U shaped clamp having spaced apart sides with inner opposed surfaces on said sides, said sides extending from a closed end to an open end, said rod position within said U shaped clamp, said first and second plane parallel sides being in engagement with said inner opposed surfaces and positioned adjacent said open end of said U shaped clamp.

UNITED STATES PATENT OFFICE IERTIFICATE OF CORRECTION p t 3,848, 106 Dated November 12, 1974 Invenmfls) lggren and Goran Boling It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the title page, after "Inventors:" delete Marshall N. Dickler Signed and sealed this 21st day of January 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM pomso uscoMM-Dc wave-PM Patent 3 848, 106 D t d November 12, 1974 Inventor) e ny ggren and Goran BOllng It is certified that error appears in t he above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the titlepage, after "Inventors:" delete Marshall N. Dickler Signed and sealed this 21st day of-January 1975.

(SEAL) Attest:

MCCOY M. GIBSON J R. v C. MARSHALL DANN Attesting Officer Commissioner of Patents oam Po-wsouo-ss) D 1 USCOMWDC 6031mm 

1. An apparatus for heating material by microwave energy comprising, a distribution element for receiving the microwave energy and coupling the microwave energy to the material to be heated, said distribution element including a dielectric material having low energy losses and having a dielectric constant exceeding the dielectric constant of air, said dielectric material having a substantially constant cross-sectional thickness at successive points therealong in the direction of propagation of the microwave energy therethrough, said dielectric material further having a thickness for maintaining the phase velocity of said microwave energy therethrouGh lower than the velocity of light in air, and having a thickness range for maintaining only the basic resonant mode of microwave energy in said dielectric material whereby the microwave energy will be concentrated within and adjacent to said dielectric material, in the immediate vicinity of the material to be heated.
 2. An apparatus according to claim 1 wherein said dielectric material maximum thickness is no greater than a thickness determined by the formula T c/2 fg Square Root Sigma - 1 wherein c is the velocity of light in air, fg is the frequency of the microwave source and Sigma is the relative dielectric constant of the dielectric material.
 3. An apparatus according to claim 2 wherein the dielectric material has the form of a plate.
 4. An apparatus according to claim 3 wherein said dielectric material has two substantially parallel surfaces forming the top and bottom surfaces respectively of said plate and further including metallic conductor means secured to said bottom surface, said top surface constructed and arranged to be placed in contact with the material to be heated.
 5. An apparatus according to claim 4 further including a second distribution element having substantially the same form of said distribution element, said distribution element and said second distribution element being positioned with respect to one another such that said dielectric material top surfaces are spaced apart by a predetermined distance and facing one another forming a plane parallel arrangement.
 6. An apparatus according to claim 2 wherein the dielectric material has the form of a cylinder.
 7. An apparatus according to claim 6 further including a core of metal positioned within said cylinder.
 8. An apparatus according to claim 2 wherein said dielectric material is cylindrical in form having an inner and outer surface, and further including metallic conductor means having a cylindrical form, said cylindrical form of dielectric material and said cylindrical form of said metallic conductor means being axially aligned with said metallic conductor means tightly encasing said dielectric material.
 9. An apparatus according to claim 2 wherein said dielectric material is shaped as a rod having first and second plane parallel sides, and further including a U shaped clamp having spaced apart sides with inner opposed surfaces on said sides, said sides extending from a closed end to an open end, said rod position within said U shaped clamp, said first and second plane parallel sides being in engagement with said inner opposed surfaces and positioned adjacent said open end of said U shaped clamp. 